The polyethylene terephthalate resin (hereinafter referred to simply as PET) has stable physical properties, high transparency, and high mechanical strength, and causes no pollution. PET is thus utilized in large quantities in various fields of application in the form of biaxially drawn, blow-molded bottles, and is quite useful especially for the bottles containing foods and drinks.
As described above, the PET bottles effectively demonstrate various excellent properties. Unfortunately, however, these biaxially drawn, blow-molded PET bottles are weak against heat and are easily deformed at a high temperature of 70° C. or more unless they are heat-treated.
For this reason, there is an urgent need for the commercialization of a highly heat-resistant PET bottle. As a method of giving heat-resisting property to the PET bottle, the applicant of this invention developed a technique called the “double blow molding process” (See Patent Publication No. 1992-56734). Bottles or containers based on this process are already in use for retort-packed foods that require heat treatment at 120° C. for 30 minutes or for other heat-treated foods.
This double blow molding technique comprises a primary blow molding step of biaxially drawing and blow molding the already molded preform of a desired shape to mold the preform into a primary intermediate product, a step of heating this primary intermediate product to shrink-mold it thermally into a secondary intermediate product, and lastly a secondary blow molding step of molding this secondary intermediate product into a bottle. When the primary intermediate product is heated and subjected to thermal shrinkage, there disappears residual distortion that has been created inside the primary intermediate product, and crystallization is promoted so that quite highly heat-resistant bottles can be obtained.
Recently, however, there is a wider range of applications than ever, not only for the containers used for the retort-packed foods that require heat treatment at a high temperature of 120° C., but also for the containers or bottles that are required to resist a heat in the range of 80-95° C. because of a necessity for heating the contents and filling them into bottles or containers. Therefore, from a point of view of production speed or energy saving, there is a need for a highly productive double blow molding process that can be used to mold heat-resistant bottles or containers.
This invention has been made to meet the above-described requirements. The technical problem of this invention is to provide a double blow molding process that can deal with various levels of heat-resistant property required for bottles. The object of this invention is to provide bottles having a necessary heat-resisting property, which can be manufactured with high productivity.